Denormalization and when to use it in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the denormalized table.

SQL

SELECT [1] FROM sales_summary;

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ACOLUMNS

BALL

C*

DEVERYTHING

Attempts:

3 left

💡 Hint

Common Mistakes

Using keywords like ALL or COLUMNS which are not valid in SELECT.

Trying to write out all column names manually.

Using * selects all columns from the table.

2fill in blank

medium

Complete the code to add a denormalized column that stores total sales.

SQL

ALTER TABLE sales_summary ADD COLUMN total_sales [1];

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ADECIMAL(10,2)

BTEXT

CVARCHAR(255)

DDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using VARCHAR or TEXT which store strings, not numbers.

Using DATE which is for dates, not numbers.

The DECIMAL(10,2) type is used for precise numbers with two decimal places, suitable for money.

3fill in blank

hard

Fix the error in the query that tries to join normalized tables but misses the join condition.

SQL

SELECT orders.order_id, customers.name FROM orders [1] customers;

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AINNER JOIN

BINNER JOIN ON

CJOIN

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using JOIN without specifying ON clause causes error.

Using INNER JOIN ON without the ON condition is invalid syntax.

The INNER JOIN keyword joins tables but requires an ON clause separately. The code is missing the ON clause, but the question asks to fix the join keyword only.

4fill in blank

hard

Fill both blanks to create a denormalized table that stores customer name and total orders.

SQL

CREATE TABLE customer_orders AS SELECT customers.name AS [1], SUM(orders.amount) AS [2] FROM customers JOIN orders ON customers.id = orders.customer_id GROUP BY customers.name;

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Acustomer_name

Btotal_amount

Corder_count

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using ambiguous or incorrect aliases that confuse the data meaning.

Using customer_id instead of customer_name for the name column.

The alias customer_name clearly labels the customer's name, and total_amount represents the sum of orders, which fits the denormalized data purpose.

5fill in blank

hard

Fill all three blanks to write a query that selects denormalized data with a condition on total sales.

SQL

SELECT [1], [2] FROM sales_summary WHERE [3] > 1000;

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Acustomer_name

Btotal_sales

Dorder_count

Attempts:

3 left

💡 Hint

Common Mistakes

Using order_count in the WHERE clause instead of total_sales.

Selecting columns that do not exist in the denormalized table.

Select the customer name and total sales columns, and filter where total sales is greater than 1000.